1. Technical Field
The present invention relates to a display apparatus capable of increasing light efficiency. More particularly, the present invention relates to a display apparatus including a display panel and a front light unit.
2. Discussion of the Related Art
Recently, as monitors, lap-top computers, TVs, and mobile communication terminals have a tendency toward lightness and slimness, a display apparatus is also required to have the lightness and slimness. In order to satisfy this requirement, various flat panel displays have been rapidly developed and extensively used instead of a conventional cathode ray tube.
A liquid crystal display (LCD) is one of the flat panel displays. The LCD includes a liquid crystal layer having dielectric anisotropy between two substrates. In the LCD, an electric field is applied to the liquid crystal layer, and the intensity of the electric field is adjusted, thereby controlling an amount of light passing through the substrates such that a desired image can be represented.
However, since the liquid crystal is not a light-emissive material and the LCD displays an image by adjusting an amount of light incident into the liquid crystal, the light needs to be radiated to the liquid crystal through a predetermined scheme.
The LCD may be classified into a transmissive LCD and a reflective LCD.
The transmissive LCD includes a backlight unit at the rear surface of a liquid crystal display panel. The transmissive LCD transmits light from the backlight unit to the liquid crystal display panel to allow an image to be displayed. The backlight unit includes a surface light source, such as a fluorescent lamp or an electro-luminescence (EL) lamp.
However, since the transmissive LCD includes the backlight unit for light emission, the transmissive LCD requires increased power consumption, and the image is not clearly viewed in a bright place such as an outdoor place exposed to direct sunlight.
The reflective LCD displays an image by using ambient light. Accordingly, the reflective LCD does not require the backlight unit, so that power consumption is low. In addition, unlike the transmissive LCD displaying an image that is not clearly viewed in a bright place, the reflective LCD can display an image that is clearly viewed in the bright place. Therefore, the reflective LCD has been applied to mobile communication terminals that are increasingly demanded in the market.
However, since the reflective LCD uses ambient light as described above, the display brightness is significantly dependent on external environments, so that an image may not be recognized in a dark place or at nighttime when there is less light. A lack of ambient light is more noticeable with a reflective LCD employing color filters or a polarizing plate, so that an auxiliary illumination device is necessary to compensate for the shortage of the ambient light.
A front-light device has been suggested as the auxiliary illumination device to supply light to the reflective LCD in a dark place. A front-light LCD with the front-light device operates similarly to the reflective LCD in a place having a sufficient amount of light, and is supplied with light from the front-light device in a place having an insufficient amount of a light. That is, the front-light LCD adopts advantages of the transmissive and reflective LCDs while removing disadvantages of the transmissive and reflective LCDs, so the front-light LCD may be expected to be extensively used.
In general, the front-light device includes a light guide plate provided at the front of a liquid crystal display panel and a light source provided at a lateral side of the light guide plate. After light output from the light source is incident into the lateral side of the light guide plate, the light travels inside the light guide plate, and then is reflected by a pattern formed on the surface of the light guide plate. The reflected light is radiated onto the liquid crystal display panel. In this case, the light is adjusted according to display information while passing through the liquid crystal display panel. Thereafter, the light is reflected by a reflective plate provided at the rear of the liquid crystal display panel, and then transmitted into the light guide plate again, so that the light is output toward a user. Accordingly, the user can recognize screen information displayed on the liquid crystal display panel even if the ambient light is insufficient.
The front-light device has been be used in a display for outdoor advertisement. In this case, strong light is incident into a display apparatus from an external light source, such as sunlight, so that visibility may be reduced.